This invention relates to improvements in so-called "infinity display" apparatus of the type used to project an image at infinity of an object displayed on a convex surface (e.g., a rear projection screen or a cathode ray tube) spaced only a short distance from an observer. Such systems are commonly employed, for exampled, in commercial and space flight simulators used to train pilots and astronauts.
In U.S. Pat. No. 3,443,858, assigned to Farrand Optical Co., there is disclosed a catadioptric optical system for providing and infinity display of an object projected onto a spherically convex diffusing screen. Such a system comprises a pair of 50% beam-splitters positioned on the optical path between the convex screen and the observer. One beam-splitter is spherically cancave, whereas the other one is planar. The combination of these elements positions the diffusing screen at the reflected position of the focus of the concave beam-splitter, in a classical catadioptric imaging system. To an observer, the virtual image of the diffusing screen now appears to be at infinity. A pair of quarter-wave plates and a pair of polarizing plates cooperates to minimize the appearance of ghost images by unwanted object light.
Optically speaking, infinity display systems of the type described above perform their intended function quite satisfactorily. Economically speaking, however, such systems tend to be relatively costly, owing primarily to the manufacturing cost associated with the relatively large aperture concave element. Such elements are commonly fabricated from a large plate of high grade glass which must be precisely ground and polished, and thereafter optically coated. The manufcturing process is slow, tedious and, hence, expensive.